1. Field of the Invention
The present invention relates to air flow measuring devices and more particularly to an air flow measuring device for measuring a flow rate of air, for example, sucked into an internal combustion engine.
2. Description of the Related Art
For example, a heating resistor type air flow measuring device is known as an air flow measuring device for measuring the flow rate of air sucked into an internal combustion engine. The air flow measuring device includes a body with a main air passage, and a measuring unit assembled to the body as disclosed in JP-A No. 14423/1999.
The measuring unit includes a flow rate detection element for measuring the flow rate of air flowing through the main air passage, an electronic circuit electrically connected to the flow rate detection element, and a housing for accommodating the flow rate detection element and the electronic circuit, the housing being fixed to the body with its tip inserted into an insertion hole of the body.
The flow rate detection element of the measuring unit is disposed in a sub-air passage into which part of air in the main air passage formed at the tip of the housing flows. Variations in positional relation between the main air passage and the sub-air passage change the distribution of flow velocity in the sub-air passage, leading to change in flow of air near the flow rate detection element, resulting in errors of measurement accuracy of the air flow measuring device.
Thus, in order to increase the accuracy of measurement of the air flow measuring device, it is important to suppress variations in assembly position of the measuring unit. Particularly, in such an air flow measuring device, variations in assembly position have great influence on measurement errors of the air flow rate as compared to any other intake air temperature sensor or pressure sensor assembled to an intake pipe of the internal combustion engine. Thus, it is very important to ensure the appropriate accuracy of the assembly position.
For example, JP-A No. 101676/1999 discloses a method for positioning a protrusion formed in a measuring unit by fitting the protrusion into a recess of a body, as a method for decreasing variations in assembly position in assembling the measuring unit to the body.